An example of the failure of Plasma Cosmology

[sjastro]

One of the claims made by plasma cosmology is dark matter is not required to explain the rotation curves for galaxies.
This is based on Anthony Peratt's computer simulation where spiral galaxies are formed by merging adjacent Birkeland currents.

An astronomer can immediately see a problem here namely the spiral arms of galaxies do not rotate even though the galaxy does.

If the arms did rotate as Peratt's model indicates since all stars have the same orbital speed, stars in the outer spiral arm having a larger orbit then stars near the centre will lag behind causing the spiral structure to tighten with each revolution until it disappears.
This is known as the winding problem.

The reason why the spiral arms do not rotate while the galaxy does is due the arms being regions of high density.
This can be explained using a traffic jam analogy.
A car entering a traffic jam will eventually exit but the traffic jam continues to exist since for every car which exits the jam is replaced by a car that enters it.

Similarly orbiting stars enter and exit these high density regions.
When dust and gas enter the high density regions they undergo compression and become regions of accelerated star formation.
These stars can ionize the surrounding gas and cause the formation of HII regions in the spiral arms which are red in colour in the galaxy image.

Peratt's model cannot explain this.

 

[Hans Blaster]

Peratt's model cannot [do] this.

Of course not. And isn't the version that appears in GIFs like this a 2D model anyway, with the currents extending infinitely in and out of the board?

I was going to talk about this in a recent thread, but it seems to have been disappeared.

 

[sjastro]

Of course not. And isn't the version that appears in GIFs like this a 2D model anyway, with the currents extending infinitely in and out of the board?

Indeed this is one of many problems.
At the most fundamental level is the existence of Birkeland currents at cosmological scales.
Birkeland currents are formed when positive ions and electrons interact with external magnetic fields such a the solar wind with the Earth's magnetosphere.
At cosmological scales where does this external magnetic field come from is the burning question.

I was going to talk about this in a recent thread, but it seems to have been disappeared.

Discussing why it disappeared may result in this thread undergoing a similar fate.
Since this is a general topic by all means feel free to provide other examples where plasma cosmology fails.

 

[Hans Blaster]

Indeed this is one of many problems.
At the most fundamental level is the existence of Birkeland currents at cosmological scales.
Birkeland currents are formed when positive ions and electrons interact with external magnetic fields such a the solar wind with the Earth's magnetosphere.
At cosmological scales where does this external magnetic field come from is the burning question.

So I may not have been quite right about the simulation shown in the GIF being 2D.

As far as I can tell the most widely distributed images of the Peratt "galaxy" model are from two papers:

A. Peratt (1986), IEEE Trans. Plasma Sci., 14, 639 "Evolution of the Plasma Universe:I. Double Radio Galaxies, Quasars, and Extragalactic Jets"
A. Peratt (1986), IEEE Trans. Plasma Sci., 14, 763 "Evolution of the Plasma Universe:II. The Formation of Systems of Galaxies"

The simulations are in 3D and feature two parallel Birkeland currents. What you see in the images are a cross-section through the currents such that they penetrate the plane of the plot. The initial currents are the round blobs seen at the beginning of the GIF. As the two current interact they twist around each other and produce simple spiral patterns in cross-section.

The most fundamental problem with the model is that they only resemble a spiral galaxy when viewed in cross-section. If viewed from another angle they would resemble two ropes twisted together with a lot of "fringe" sticking out from them.

The cross-section bears a resemblance to the stars in a spiral galaxy only because Peratt used a particle method (nothing wrong with that) where the plasma was broken up into "blobs" that each represent a certain amount of matter and their motion was simulated (again nothing wrong with the general method). His plots don't show where the cross-section glows brightest, but rather where the mass is in that particular plane.

Of course, we know that spiral galaxies (at least the spiral parts) are geometrically thin. This comes from the observations of thousands of external spiral galaxies with various orientations *and* from measuring the structure of our own galaxy, which is *definitely* thinner normal to the galactic plane than the size of that plane. From external galaxies we can tell that the typical spacing between spirals is much larger than their widest dimension. (Yes, I know there are interacting pairs and other small groups, but none of these look like a series of thin "lit" regions on the same "rope".)

Peratt's model provides *no* reason why only a thin section of the "intergalactic currents" are lit-up. The lab counterparts (which probably inspired the galactic comparison and simulation) glow all along the current "ropes", not just in thin regions. There is also an extensive discussion of the radio emission from these plasmas, but they should be emitting along their lengths, not just in the thin "galaxies".

 

[sjastro]

So I may not have been quite right about the simulation shown in the GIF being 2D.

As far as I can tell the most widely distributed images of the Peratt "galaxy" model are from two papers:

A. Peratt (1986), IEEE Trans. Plasma Sci., 14, 639 "Evolution of the Plasma Universe:I. Double Radio Galaxies, Quasars, and Extragalactic Jets"
A. Peratt (1986), IEEE Trans. Plasma Sci., 14, 763 "Evolution of the Plasma Universe:II. The Formation of Systems of Galaxies"

The simulations are in 3D and feature two parallel Birkeland currents. What you see in the images are a cross-section through the currents such that they penetrate the plane of the plot. The initial currents are the round blobs seen at the beginning of the GIF. As the two current interact they twist around each other and produce simple spiral patterns in cross-section.

The most fundamental problem with the model is that they only resemble a spiral galaxy when viewed in cross-section. If viewed from another angle they would resemble two ropes twisted together with a lot of "fringe" sticking out from them.

The cross-section bears a resemblance to the stars in a spiral galaxy only because Peratt used a particle method (nothing wrong with that) where the plasma was broken up into "blobs" that each represent a certain amount of matter and their motion was simulated (again nothing wrong with the general method). His plots don't show where the cross-section glows brightest, but rather where the mass is in that particular plane.

Of course, we know that spiral galaxies (at least the spiral parts) are geometrically thin. This comes from the observations of thousands of external spiral galaxies with various orientations *and* from measuring the structure of our own galaxy, which is *definitely* thinner normal to the galactic plane than the size of that plane. From external galaxies we can tell that the typical spacing between spirals is much larger than their widest dimension. (Yes, I know there are interacting pairs and other small groups, but none of these look like a series of thin "lit" regions on the same "rope".)

Peratt's model provides *no* reason why only a thin section of the "intergalactic currents" are lit-up. The lab counterparts (which probably inspired the galactic comparison and simulation) glow all along the current "ropes", not just in thin regions. There is also an extensive discussion of the radio emission from these plasmas, but they should be emitting along their lengths, not just in the thin "galaxies".

Peratt’s paper, “IEEE Trans. Plasma Sci., 14, 639 Evolution of the Plasma Universe:I. Double Radio Galaxies, Quasars, and Extragalactic Jets" can be put to the test with regards to Double Radio Galaxies since are a few candidate objects are close enough for the lobes or jets to be imaged and analysed in detail.
According to Peratt the lobes or jets are Birkeland currents flowing through the galaxy.

First the image of a Double Radio Galaxy Hercules A;

What is immediately striking about this image it creates the impression of symmetry suggesting the lobes are created by material ejected from the galaxy rather than a Birkeland current flowing through the galaxy.

Double radio galaxies have the following general structure;

This is consistent with the explanation that at the centre of the galaxy is a supermassive black hole being fed by an accretion disk.
High energy positive ions and electrons are emitted as jets close to the speed of light beyond the event horizon in both directions perpendicular to the accretion disk.
When passing through a magnetic field the electrons radiate synchrotron radiation in the radio frequency range.
The jets interact with the surrounding medium which is dragged along by the jets and forms the wake.
Since the jets travel at high velocities a shock front is formed at the ends of the jet.
The hot spot is where the jet is brought to a stop and the rapid deceleration associated with this is also a source of synchrotron radiation.

The mechanism proposed by Peratt cannot explain these observations.

 

[sjastro]

There are various plasma cosmology models, the most famous is the Alfven-Klein model.
One of the problems with Big Bang cosmology is explaining the matter-antimatter asymmetry.
According to Big Bang nucleosynthesis equal amounts of matter and antimatter should have been produced in the early universe which should still exist today.

The Alfven-Klein model postulates there is no asymmetry and the Universe is composed of cells of matter or antimatter which are separated by a boundary layer.
The boundary layer is not an impermeable barrier and there can be interaction between matter and antimatter.

According to Alfven in our observable universe there has been a mixing of matter and antimatter to form an ambiplasma.
Alfven realised a static universe would collapse under gravity and when the ambiplasma filled observable universe collapsed to around 10 times the Hubble distance c/H₀ where H₀ is Hubble’s constant, the density is sufficiently high for matter and antimatter to react to not only arrest contraction but eventually cause expansion as observed today.
While Alfven is ambiguous on whether the universe is infinitely old in the above link there are no doubts here in his attack on the Big Bang.

What are observations which contradict the Alfven-Klein model?
First of all when matter and antimatter annihilate each other high energy γ photons are produced.
Our observable universe should be awash with these high energy photons but this is not observed.
Secondly the Cosmic Radiation Background cannot be explained by this model.

Thirdly since the model is based on the universe being infinitely old, the cosmic chronometers ²³⁵U, ²³⁸U, ²³²Th and ²⁴⁴Pu should not exist.
While shorter half life, lower mass number radioactive nuclei are replenished by processes such as fusion in stellar cores, cosmic ray fission, radioactive chain decay etc, cosmic chronometers are exclusively produced in rapid neutron capture process by nuclei initiated by merging neutron stars and will eventually vanish.

²³⁸U is shown as U II in the spectrum of a 13 billion year old star.

This spectrum cannot exist in the Alfven-Klein model.

 

[Hans Blaster]

While Alfven is ambiguous on whether the universe is infinitely old in the above link there are no doubts here in his attack on the Big Bang.

Ahh, from the "grumpy old man" phase of his career.

Perhaps of note for this website, Alfven criticizes the Big Bang model for its religious implications, motivations, and origins as creation ex nihlio. I have seen his distant followers (though he would likely reject them as do his acolytes) on other sites criticize the BB model as religious.

The tone of this article is likewise quite unprofessional.

 

[sjastro]

Ahh, from the "grumpy old man" phase of his career.

Perhaps of note for this website, Alfven criticizes the Big Bang model for its religious implications, motivations, and origins as creation ex nihlio. I have seen his distant followers (though he would likely reject them as do his acolytes) on other sites criticize the BB model as religious.

The tone of this article is likewise quite unprofessional.

The unprofessional tone has become the template for criticizing Big Bang Cosmology today by various groups.
George Lemaitre being a Jesuit Priest is the focus for the religious implications yet from a historical perspective Big Bang Cosmology is also the work of Alexander Friedmann, Howard Robertson and Arthur Walker.

 

[sjastro]

Plasma cosmology has struggled to explain the nature of the CMB (cosmic radiation background).
Attempts to explain the temperature of space by the radiation of stars as described by Eddington resulted in a calculated temperature of 3.2K.
Under these conditions a blackbody spectrum which is a Big Bang theory prediction cannot be replicated.
While an individual spectrum of a star is a good approximation of a blackbody spectrum at a given temperature, when the combined radiation of stars at different colour temperatures is taken into consideration, the resulting spectrum for space is a not a blackbody as it will have a much broader peak than the blackbody spectrum of the CMB.

ISD is the temperature of space with interstellar dust which Eddington did not take into account.
The vertical line at 2.64mm is the CN (cyano radical) transition.
CN is a radical that rotates in space in quantized rotation levels.
It absorbs microwave radiation at 2.64mm where the molecule undergoes a rotation level transition.
This occurs at very low temperatures of around 2.3K.
Where this line intersects the CMB and Eddington curves the intensity of the Eddington curve at 2.64mm is around 700 million times smaller than the CMB curve.
The similarity between the CMB temperature of 2.7K and the Eddington temperature of space is purely coincidental.

Eric Lerner the plasma cosmologist theorized the CMB is thermalised and isotropized by photons undergoing synchrotron absorption and emission instead of a surface of last scattering which is the boundary when the Universe went from being opaque to transparent.

https://www.researchgate.net/publication/227122671_Intergalactic_Radio_Absorption_and_the_COBE_Data

Synchrotron radiation occurs when charged particles such as electrons are accelerated in magnetic fields.
The claim is the spectrum of the synchrotron emitted photons matches the blackbody curve of the CMB.
Lerner's model is based on the COBE data where the anisotropic details are faintly seen due to the low resolution.
Whether the model was still relevant when using the higher resolution WMAP and Planck data which reveal the very small anisotropic temperature variations in the CMB is not obvious, but there is an observation which contradicts Lerner’s model.
If photons are emitted due to synchrotron radiation the photons will exhibit linear polarization if the axis of the helical motion of the electron is perpendicular to the observer, or circular polarization for any other orientation.

Big Bang theory states the surface of last scattering involves Thomson scattering where photons are exclusively linear polarized.
This is supported by observation.

 

[sjastro]

A serious flaw in Plasma Cosmology is there is no unique explanation for redshift.
Whereas in Big Bang Cosmology there are two types of redshift, Cosmological and Doppler redshift which are consistent with observation, the French cosmologist and astrophysicist Henri Reboul found over thirty categories of redshift theories proposed in over 200 papers, which could apply to Plasma Cosmology.

Rather than going through the merits of each theory all these redshift theories are found to be contradicted by observation.
The observation is Thomson scattered photons from the CMB as discussed in my last post.
Thomson scattering occurs where the energy of the incident photons is much less than the rest mass of electrons in the plasma and not only become polarized but do not lose energy and are not redshifted as the collisions are elastic.
Thomson scattering only occurs at a 10% level in the CMB, the remainder are not scattered.
If there is an underlying plasma cosmology redshift theory then Thomson scattered photons should not be observed.
The observed redshift is a Cosmological redshift which does not involve any physical interaction between plasma and photons.

This does raise the question if Thomson scattering is observed and is the low energy limit for Compton scattering of photons which is inelastic and involves energy loss, then why is Compton scattering not a satisfactory replacement for cosmological redshift?

This is the “classical” version of Compton scattering.
The photon is scattered at an angle θ to the path of the incident photon and this would be observed as blurring of distant objects such as galaxies.
Since distant galaxies are not blurred this is the standard argument against Compton scattering.

There is however an even more fundamental reason why Compton scattering doesn’t work.
First of all here is a brief intro on Feynman diagrams which is a pictorial representation of quantum mechanical interactions involving particles and photons and will serve to illustrate later on in the post why Compton scattering won't work.

The Feynman diagram for Compton scattering in terms of photon momentum K and K’ and electron momentum p and p’ is shown;

From this diagram can be derived the ratio P of the photon energy after and before the collision.

The bulk of astronomy is done in the radio to the low frequency UV range
(4 x 10⁻¹³ eV – 50 eV ) and given the rest mass of an electron is 0.51 MeV, the term
(Eγ/mₑc²) ≈ 0 in the equation and therefore P = 1 in which case there is no energy loss and the scattering is either Thomson or no scattering occurs.

 

[sjastro]

In a previous post it was shown the existence of cosmochronometers cannot be explained by Plasma Cosmology.
An extension of this problem is the issue of entropy or randomness.
The entropy of the Universe increases with time and will eventually reach a state of thermal equilibrium.
In this state there is no energy available to perform work for star formation while most existing stars will end up as black dwarfs.
The Universe will end up as a cold dark place.

This is a problem faced by Plasma Cosmology where the Universe in infinitely old and therefore “has had the time” to reach thermal equilibrium.
While the increase in entropy in the Universe is an irreversible process, attempts have been made to explain at local scales the process may be reversible in some cases where entropy does not increase.

This can be explained with the sand castle analogy.
A sand castle is in a highly organized state of low entropy unlike the surrounding sand.
A gust of wind reduces the sand castle to a heap which is now in a state of higher entropy.
Suppose there is another gust of wind; what is the likely outcome?
One would expect the gust of wind to level the heap of sand further increasing its entropy but there is the “possibility” the sand castle is reformed where each grain of sand ends up in exactly same initial position before the first gust of wind.
Needless to say this is highly improbable and made even further unlikely if the grains of sand, in this case protons p⁺, are predicted to decay according to particle physics GUT (Grand Unified Theory) according to the equations.

p⁺ → e⁺ + π ͦ
π ͦ → 2γ

e⁺ is a positron, π ͦ a neutral pion and γ is a photon.
The half life for proton decay is theorized to be at least 1.67 x 10³⁴ years and is therefore “young enough” to disappear in an infinitely old Universe.

 

[FrumiousBandersnatch]

In a previous post it was shown the existence of cosmochronometers cannot be explained by Plasma Cosmology.
An extension of this problem is the issue of entropy or randomness.
The entropy of the Universe increases with time and will eventually reach a state of thermal equilibrium.
In this state there is no energy available to perform work for star formation while most existing stars will end up as black dwarfs.
The Universe will end up as a cold dark place.

This is a problem faced by Plasma Cosmology where the Universe in infinitely old and therefore “has had the time” to reach thermal equilibrium.
While the increase in entropy in the Universe is an irreversible process, attempts have been made to explain at local scales the process may be reversible in some cases where entropy does not increase.

This can be explained with the sand castle analogy.
A sand castle is in a highly organized state of low entropy unlike the surrounding sand.
A gust of wind reduces the sand castle to a heap which is now in a state of higher entropy.
Suppose there is another gust of wind; what is the likely outcome?
One would expect the gust of wind to level the heap of sand further increasing its entropy but there is the “possibility” the sand castle is reformed where each grain of sand ends up in exactly same initial position before the first gust of wind.
Needless to say this is highly improbable and made even further unlikely if the grains of sand, in this case protons p⁺, are predicted to decay according to particle physics GUT (Grand Unified Theory) according to the equations.

p⁺ → e⁺ + π ͦ
π ͦ → 2γ

e⁺ is a positron, π ͦ a neutral pion and γ is a photon.
The half life for proton decay is theorized to be at least 1.67 x 10³⁴ years and is therefore “young enough” to disappear in an infinitely old Universe.

Yes; there is another way for local entropy reductions to occur, e.g. chemistry producing complex molecules, but that requires a plentiful surplus of free energy to overcome energy barriers and to form bonds, etc., and there will be no free energy in an infinitely old universe at thermal equilibrium (AIUI, in a metaverse such as that of eternal inflation, this only applies to the individual universes that it produces).

 

[sjastro]

In an earlier post Anthony Peratt’s theory that radio double lobes emanating from galaxies were Birkeland currents was shown to be refuted by observation.

The general claim made by plasma cosmologists is the cosmic web filaments in this computer simulation using observation data are Birkeland currents is also refuted by observation.

In this case it is the presence of the 21 cm hydrogen line.
The hydrogen atom is composed of a nuclear spin I = ½ and an electron spin s = +/- ½.
The hydrogen electron is in a 1s orbital and the energy level of this ground state can split into two levels where the nuclear and electron spins are parallel or anti-parallel.

The 21 cm line is where electromagnetic transitions can occur for neutral hydrogen.
Radio telescopes reveal the 21 cm line in the filaments indicating they are composed of neutral hydrogen not Birkeland currents.

There is an emerging science known as 21 cm Cosmology which not only refutes Plasma Cosmology but provides supportive evidence for Big Bang Cosmology particularly the abundance of hydrogen in the early Universe.
A simplified and abbreviated version is shown in this video.

 

[Hans Blaster]

In an earlier post Anthony Peratt’s theory that radio double lobes emanating from galaxies were Birkeland currents was shown to be refuted by observation.

The general claim made by plasma cosmologists is the cosmic web filaments in this computer simulation using observation data are Birkeland currents is also refuted by observation.

In this case it is the presence of the 21 cm hydrogen line.
The hydrogen atom is composed of a nuclear spin I = ½ and an electron spin s = +/- ½.
The hydrogen electron is in a 1s orbital and the energy level of this ground state can split into two levels where the nuclear and electron spins are parallel or anti-parallel.

The 21 cm line is where electromagnetic transitions can occur for neutral hydrogen.
Radio telescopes reveal the 21 cm line in the filaments indicating they are composed of neutral hydrogen not Birkeland currents.

There is an emerging science known as 21 cm Cosmology which not only refutes Plasma Cosmology but provides supportive evidence for Big Bang Cosmology particularly the abundance of hydrogen in the early Universe.
A simplified and abbreviated version is shown in this video.

When you watch the video, at the end, the presenter shows the frequency of the absorption feature from the 21 cm line in the background at 78 MHz. This corresponds to 1+z = 18 (or z=17) that is exactly within the predicted range for the onset of formation for the first stars.

 

[sjastro]

The SKA (Square Kilometre Array) the biggest science project in history is scheduled for construction starting this year will be used to map the cosmic web.

The Square Kilometer Array will be the first telescope capable of directly mapping the full extent of the cosmic web that characterizes the large scale structure of the Universe. The approach highlighted here is to map the cosmic web via the detection of diffuse synchtrotron emission associated with the growth of the large scale structure in the Local Universe. As matter collapses onto the filaments of the web it will be shock accelerated to extremely high energies. In the presence of even extremely weak magnetic fields these accelerated particles will emit large scale, diffuse synchrotron emission. The tremendous sensitivity of the SKA over a wide field and at low frequencies will allow us to fully map the cosmic web in the local Universe.

Here is some info (albeit a promotion) on the SKA.

 

[sjastro]

Ahh, from the "grumpy old man" phase of his career.

Perhaps of note for this website, Alfven criticizes the Big Bang model for its religious implications, motivations, and origins as creation ex nihlio. I have seen his distant followers (though he would likely reject them as do his acolytes) on other sites criticize the BB model as religious.

The tone of this article is likewise quite unprofessional.

The modern day expression of the “them versus us” attitude is found in this plasma cosmology video.

It’s interesting how the manner of response from each of the professors is rated first followed by their objections to Plasma Cosmology.

To digress you don’t have to be a nice guy to win a Nobel Prize.
By all accounts Wolfgang Pauli of Pauli Exclusion Principal fame was one such individual.

Pauli was the first to coin the phrase “not even wrong”.

This paper is so bad it is not even wrong.

His comments on the classic work “Elementare Quantenmechanik” by the Nobel Prize winner Max Born and Pascual Jordan.

The setup of the book as far as printing and paper are concerned is splendid.

Then there is this about a colleague.

I don't mind your thinking slowly; I mind your publishing faster than you think.

Freeman Dyson expressed the attitude towards Pauli.

What distinguished him mostly was his sharp tongue. Everybody was scared of him. He had nasty things to say about almost everybody……

Yet despite not winning any popularity contests he was respected.
From Nobel Prize winner Steven Weinberg.

Although Heisenberg and Dirac may have been even more creative than Pauli, no physicist alive was more clever.

 

[Hans Blaster]

The modern day expression of the “them versus us” attitude is found in this plasma cosmology video.

It’s interesting how the manner of response from each of the professors is rated first followed by their objections to Plasma Cosmology.

To digress you don’t have to be a nice guy to win a Nobel Prize.
By all accounts Wolfgang Pauli of Pauli Exclusion Principal fame was one such individual.

Pauli was the first to coin the phrase “not even wrong”.


His comments on the classic work “Elementare Quantenmechanik” by the Nobel Prize winner Max Born and Pascual Jordan.


Then there is this about a colleague.


Freeman Dyson expressed the attitude towards Pauli.


Yet despite not winning any popularity contests he was respected.
From Nobel Prize winner Steven Weinberg.

Who is the author of the video? It doesn't sound like the thunderbolts people. I haven't seen the "movie" he speaks of (and have no desire to watch it). Arrogant and insulting fellow isn't he.

He seems rather defensive of Peratt and makes a claim I've never heard before that Peratt was "blowing up nukes" at Los Alamos. From years of hearing the hagiography of Peratt from the PC/EU followers, I'm surprised that I never heard that. Now a lot of people at LANL do open and classified work, so it's entirely possible. It's also possible that the narrator is assuming so incorrectly.

 

[Hans Blaster]

Who is the author of the video? It doesn't sound like the thunderbolts people. I haven't seen the "movie" he speaks of (and have no desire to watch it). Arrogant and insulting fellow isn't he.

I did track back the YT to the channel and I think I found the "movie" in question. I don't think we can take anything seriously that clips 5 minutes of P-M. Robataille (sp) rambling on about how black bodies aren't real and therefore the CMB isn't actually detected.

 

[FrumiousBandersnatch]

The modern day expression of the “them versus us” attitude is found in this plasma cosmology video.

It’s interesting how the manner of response from each of the professors is rated first followed by their objections to Plasma Cosmology.

Interesting to see that their defence against a serious hole in PC (the H, He, Li ratios) was whataboutism, "Yeah, well the standard model has big problems too..." Rather reminds me of Michael

 

[Hans Blaster]

Interesting to see that their defence against a serious hole in PC (the H, He, Li ratios) was whataboutism, "Yeah, well the standard model has big problems too..."

Plus "this problem only exists if we see everything". Whatever, dude. Yes helium is hard to detect, but when it can be detected it falls within the abundance predicted by Big Bang nucleosynthesis. No one ever sees 15% He (by mass) anywhere in stars or interstellar or intergalactic gas when it can be clearly detected.